Ion implanters are commonly used in the production of semiconductor wafers. An ion source is used to create a beam of positively charged ions, which is then directed toward the workpiece. As the ions strike the workpiece, they change the properties of the workpiece in the area of impact. This change allows that particular region of the workpiece to be properly “doped”. The configuration of doped regions defines the functionality of the workpiece, and through the use of conductive interconnects, these workpieces can be transformed into complex circuits.
In many applications, the ion beam is directed so as to strike the workpiece in a direction normal to the plane of the workpiece. FIG. 1 shows a representative orientation of workpiece 100 in three dimensions, X, Y and Z. In many applications, the ion beam (not shown) is directed toward the workpiece 100 in a direction substantially parallel to the Z axis. In this manner, the ion beam is substantially perpendicular to the workpiece in both the X and Y axes. The ion beam can be thought of as a set of ion beamlets, each beamlet comprising a single line in the XZ plane. While it is important that the entire beam is substantially perpendicular to the workpiece, it is equally important that each of the individual beamlets is also perpendicular to the workpiece in both the X and Y axes. FIG. 2a shows an ion beam 200 which is made up of a plurality of beamlets 210. Although only several are shown for purposes of illustration, the ion beam can be comprised of an arbitrary number of beamlets. In this Figure, all of the beamlets 210 are parallel to one another. In contrast, FIG. 2b shows an ion beam 250, which is also substantially perpendicular to the workpiece. However, its component ion beamlets 270 are not parallel to one another, and consequently some of these beamlets are not perpendicular to the workpiece.
Since an ion beam is a three dimensional entity, parallelism exists in several dimensions. For example, ion beamlets can be parallel across the X dimension (i.e. the XZ plane). In this dimension, deviations in the Y dimension are not considered. Similarly, ion beamlets can be parallel across the Y dimension (i.e. YZ plane), where deviations in the X dimension are not considered. It should be obvious to one of ordinary skill in the art that the ion beamlets of an ion beam can display parallelism in one dimension (i.e. width) without displaying parallelism in the orthogonal dimension (i.e. height).
As geometries continue to shrink, it is critical to be able to measure the orientation of the ion beam, and to control the beam so as to maximize the degree of parallelism. In measuring an ion beam, there are several important parameters. The first is the divergence of the ion beam. As used in this disclosure, the term “divergence” is defined as the mean angle of incident of the ion beam onto the workpiece. This is determined to be the mean of the individual ion beamlet's angles of incidence. The second parameter is “spread”, which is defined as the range of these mean angles. These parameters help define the ion beam. For example, FIG. 3 shows an ion beam having a divergence of 10° and a spread of 0°. This low spread value indicates that the individual ion beamlet have a high degree of parallelism. The divergence value indicates that the ion beam is striking the workpiece at an 80° angle, rather than perpendicularly. FIG. 4 shows an ion beam having a divergence of 0° and a spread of 10°. In this case, the ion beam on average strikes the beam perpendicularly, but the angles of incidence of the individual beamlets differ by 10°. Thus, although the ion beam as an entity is perpendicular to the workpiece, the individual beamlets differ greatly in angular direction. For example, the outermost beamlets in FIG. 4 vary in incidence angle by 10°. It should be noted that it is much easier to compensate for variations in divergence in the ion implantation system than angular spread. In many applications, the workpiece is simply rotated about the Y axis so that the desired mean incident angle is obtained. Angular spread is corrected by adjustments to the beamline components. For example, the mass analysis magnet, the scanner, the correction magnet, D2 focus, the rods and poles, D1 and D2 suppression, and the various quadrupole coils can all be adjusted to vary, and minimize the angular spread of the ion beam.
To perform these adjustments, it is imperative to be able to measure very accurately the incidence angle of the ion beam. Numerous patents disclose apparatus and methods of determining these parameters for an ion beam. Unfortunately, many of the techniques currently used are time consuming and relatively inaccurate. The time required to measure the ion beam negatively impacts the time to calibrate the beam. This hurts the overall system utilization. The inaccuracy of the measuring devices reduces the ability to perfectly control the ion beam as well.
It would be greatly beneficial to be able to measure these ion beam characteristics to within +/−0.1°. It would also be very advantageous if these measurements could be made quickly, so that real time tuning for minimum angle spread is possible.